1. Field of the Invention
This invention relates generally to a corona discharge ignition system including an igniter for emitting a non-thermal plasma, and more specifically to a firing tip of the igniter.
2. Related Art
An example of a corona discharge ignition system is disclosed in U.S. Pat. No. 6,883,507 to Freen. The corona discharge ignition system includes an igniter with an electrode charged to a high radio frequency voltage potential, creating a strong radio frequency electric field in the combustion chamber. The electric field causes a portion of a mixture of fuel and air in the combustion chamber to ionize and begin dielectric breakdown, facilitating combustion of the fuel-air mixture. The electric field is preferably controlled so that the fuel-air mixture maintains dielectric properties and corona discharge occurs, also referred to as a non-thermal plasma. The ionized portion of the fuel-air mixture forms a flame front which then becomes self-sustaining and combusts the remaining portion of the fuel-air mixture. Preferably, the electric field is also controlled so that the fuel-air mixture does not lose of all dielectric properties, which would create a thermal plasma and an electric arc between the electrode and grounded cylinder walls, piston, or other portion of the igniter, referred to as power-arcing.
The igniter of the corona discharge ignition system typically includes an electrode having an electrode body portion extending longitudinally from an electrode terminal end receiving the high radio frequency voltage, along an electrode center axis, to an electrode firing end. The electrode may include a firing tip adjacent the electrode firing end for emitting the radio frequency electric field. The firing tip is symmetric relative to the electrode center axis. The igniter of the corona discharge ignition system does not include any grounded electrode element in close proximity to the firing tip. Rather, the ground is provided by the cylinder walls or the piston of the internal combustion engine. An example of a corona igniter with a symmetric firing tip is disclosed in U.S. Patent Application Publication No. US 2010/0083942 to Lykowski and Hampton.
In internal combustion engine systems, especially non-homogeneous combustion systems, like gasoline direct ignition systems, placement of the ignition source relative to the fuel-air mixture is critical to a robust combustion. In certain engine applications, the fuel is provided to the combustion chamber as a spray, but the spray is typically too rich in fuel to ignite directly and may be flammable only at the outside edges of the spray, where the fuel mixes with the air of the combustion chamber. Accordingly, the igniter must be spaced from the fuel injector so that the firing tip is disposed in a predetermined location relative to the outside edge of the fuel spray. The igniter is also preferably spaced from the fuel spray to prevent erosion and corrosion caused by the fuel spray. However, if the igniter is too close to the cylinder walls or piston, power arcing may occur between the firing tip and the cylinder walls or piston, which would eliminate any corona discharge and could be detrimental to combustion. Further, the fuel injector oftentimes cannot be moved from a central location in the combustion chamber, which further complicates the system design.